Electromagnetic induction apparatus and method of making the same



ay 30, 1939- .1. c. GRANFI E LD 2.160588 ELECTROMAGNETIC INDUCTIONAPPARATUS AND METHOD OF MAKING THE SAME Filed Jan. :50, 1937 3Sheets-Sheet 1 Fig.1.

Inventor John C. Gfianfield His Attovneg.

' M J. c. GRANFIELD 2,160,588

ELECTROMAGNETIC- INDUCTIQI APPARATUS AND'METHOD OF MA KING THE SAMEFiled Jan. 30, 19.37 3 Sheets-Sheet 2 I A l Inventor John C. Granf'ieldb Jaw M 8 His A1313 ornea -May 30, 1939-- J. c. GRANFIELDELECTROMAGNETIC INDUCTION APPARATUS AND METHOD OF MAKING THE SAME FiledJan. 30, 1937 s Sheets-Sheet 3 Inventor: John C. Gvanfield,

b W I is Attovneg.

Patented May 30, 1939 UNITED STATES PATENT OFFICE ELECTROMAGNETICINDUCTION APPARA- TUS AND METHOD OF MAKING THE SAME New York ApplicationJanuary 30, 1937, Serial No. 123,249

22 Claims.

This application is a continuation-in-part of my application Serial No.77,499, filed May 2, 1936, for Transformer core, relating to the methodof making electrical induction apparatus such astransformers'andreactors comprising a magnetic core and one or more conductive windings.

Most transformers except those for high frequency radio applicationscomprise a magnetic core on which there is at least one primary windl0ing and one secondary winding. There maybe more than one primary windingand more than one secondary winding. A reactor ordinarily comprises amagnetic core and a windin on the core, the device providing anelectromo ve force of self induction for a wide variety of applications.There may be other. windings on the core which in some cases may besupplied with direct current for saturating purposes.

My invention relates to an improved construction of such electromagneticinduction apparatus which enables the cost to be greatly reduced withoutsacrificing desirable operating characteristics. While the principles ofmy invention are applicable to practically all types and ratings oftransformers including power transformers, X-

ray transformers, etc., my improvements have their greatest economicalimportance at present in the case of distribution transformers,especially in the range from 1 to 50 kw. capacity,

such transformers being used in enormous numbers. My improvements arealso applicable with substantial savings to instrument transformers, tosmall transformers such as radio transformers and small transformers forgeneral application on standard commercial frequencies. My improvementslend themselves also to economical constructions for much higherfrequencies.

Distribution transformers have been developed to a very high state ofperfection, particularly by reason of improvements in the quality of themagnetic core material. Hadfields invention of silicon-iron alloy made aradical improvement in transformers about thirty years ago, reducing thewatt losses in the core about 30%- as compared with iron or soft steelpreviouslylused, the sillcon-iron alloy producing a reduction in bothhysteresis'and eddy current losses. For thirty years or more thestandard construction of distribution transformers has involved buildingup the magnetic core of thin laminatlons punched or stamped toappropriate sizes. The presence of silicon increased the diillculty ofrolling and, in the early days, it was difllcult to roll a sheet with asilicon cgntent of as much as 3%, but with no experience in rolling andheat-treating, the silicon content has been gradually raised until it isnow from 4 to 5% in transformer sheets, and has been of the order of 3to l for about the last twenty years. The reduction in watt losses insilicon steel from about 1.25 (at 10,000 gausses and a frequency ofcycles) at the time it was first used extensively, to about .5 watt perpound at the present time is in a measure attributed to the increase insilicon content which has increased the resistivity of the iron and soreduced its eddy current losses and, by its chemical action on thedissolved carbon the silicon also reduced the hysteresis loss. Thelarger part of the reduction in watt loss was, however, accomplishedthroughreflnements in the process of making the steel and a betterunderstanding and control of the physical structure of the materialeflected by improved rolling and annealing methods. One of the latestdevelopments along this line of increasing the silicon content is the 6to 6V sili-v con steel disclosed in Ruder application Serial No. 97,678,filed August 24, 1936, assigned to the-same assignee as'the presentapplication, and also disclosed in British Patent 380,387 of. 1932 ofthe British Thomson-Houston Company, Ltd. Since the filing of thepresent application the Ruder application has resulted in United StatesPatent No. 2,088,440, issued July 27, 1937.

This increase in silicon content, together with improvements in thetechnique of hot rolling and annealing, have resulted in increasedpermeability, as well as reduced watt losses in the hot rolled product,permitting a constant'gain in watt losses and an improvement inmagnetizing current characteristics of distribution and powertransformers so long as they were designed tooperate at flux densitiesnot exceeding 10,000 to 12,000 gausses. The increased permeability hasbeen in the lower range of magnetic densities, and with higherdensities, the amount of magnetizing current required increases sorapidly as to overbalance any gain from reduced watt losses. A fluxdensity of 10,- 000 gausses has therefore become the standard referencepoint for watt losses in transformer sheet steel specifications.

The loss in transformer steel is partly a hysteresis loss and partly aneddy current loss. The ratio of these losses varies with the siliconcontent, the

eddy current loss increasing as the silicon content is lowered anddecreasing as the silicon content is increased by reason of the factthat the resistivity of the material increases with the silicon content.At normal densities, that is densities of about 10,000 gausses, thehysteresis loss in hot rolled 455% silicon transformer steel is aboutand the eddy current loss about 25% of the total. The ratio is alsoaffected by the thickness of the sheet used, the eddy current loss inthe apparatus being reduced as the thickness of the sheet is reduced.With thin sheets, the eddy current loss decreases roughly in proportionto the square of the thickness; that is, by decreasing the thickness ofthe sheets by one-half, the eddy current loss in a device such as atransformer may be decreased to about one-fourth. In the present daydevelopments of hot rolled sheets, however, a limit is reached in thedirection that one may go in reducing thickness. Below a thickness of 10or 11 mils, the total watt losses are increased because of the increasein hysteresis loss which is probably due to a breaking up of the grainof the material, the result being that 14 mil sheets have usually beenadopted as standard in the hot rolled high grade material because oflimitations in thickness tolerance necessary to avoid producing sheetsbelow a thickness which would increase hysteresis loss.

Moreover, where the cores of electromagnetic induction apparatus such astransformers, reactors, and the like, are built up of punched or stampedpieces, as in present day standard practice, there are a number of otherconsiderations which militate against going to a thinness below about 14mils. The hot rolled sheets are customarily made by thepack-rollingprocess producing sheets about 10 feet long and of a widthof about 30 inches, although wider, as well as narrower, sheets may bemade. The thinner the sheet, the more likely the sheets are to sticktogether during annealing and the more diflicult it is to handle thesheets without distorting them and the thinner the materialis, thegreater is the number of sheets required for any given volume ofmateriaL The punching of laminations from sheets as they come from themill is expensive in handling and is wasteful because of the scrap leftafter the punching operation. It has therefore been customary withcertain large transformer manufacturers for many years to cut the sheetsintostrips of the desired width, weld them end to end to make acontinuous strip, and then run such strip through the punching machine.The punched out laminations after annealing are then ready to beassembled around the windings of the transformers or reactors for whichthey have been punched. The thinner the material used, the greater isthe tendency of the laminations to stick together when annealed and thegreater the number of laminations which must be handled and stacked, allof which adds to the cost of manufacture. It has been found byexperience that a reduction in the thickness of the punched outlaminations with a corresponding increase in their number, causes anincrease in magnetizing current which seems to correspond to increasedair gap reluctance.

The present hot rolled silicon steel sheet material depends for itsdesirable characteristics upon relatively large crystals. Tests showthat there is some difference in permeability in the hot rolledmaterial, depending upon whether the flux flows through the sheetin onedirection or another, but since there is no very pronounced favorablemagnetic orientation of the grain of the material, the difference in thelosses, depending upon whether the flux flows in the most favorabledirection or at right angles hereto, is only of the order of 10% to 15%.It is common to punch out the laminations from the hot rolled strip intheform of L-shaped punchings, each armpf the 24, 1933, and Goss1,965,559, July 3, 1934. The

hot rolled material at the 45 degree angle are about as good as at themost favorable angle. The use of L-shaped instead of straight punchingsreduces the air gap reluctance, since each L- shaped punching eliminatesone corner air gap in the built-up core. It is therefore possible bypunchings of such L shape from such hot rolled material to secure anactual gain in the magnetizing current and watt loss characteristics ofthe complete apparatus, as well as reduce the number of punchings whichhave to be made and assembled. However, in any structure built up fromlaminations, it is impossible to take full advantage of the magneticproperties of the material because, even if the laminations be punchedso as to have the most favorable orientation of the grain longitudinallyof the laminations, the direction of the flux at the corners will alwaysbe at an angle varying from zero to to the direction of the grain. Thepunching of the laminations so as to have the favorable grainorientation longitudinally of each lamination requires that there be anair gap at each corner of the built-up core.

It is an object of my invention to provide a construction wherein fulladvantage is taken of the favorable magnetic orientation of the grainwhile at the same time air gap reluctance is enormously reduced. It is afurther object of my invention to provide a method of construction whichmakes it possible economically to use material having a pronouncedfavorable orientation of the grain and greatly reduce the cost of theapparatus while obtaining very desirable opcrating characteristics fromthe standpoint of watt loss and magnetizing current. While my method ofassembly, hereinafter described in detail, is particularly applicable'to material such as high reduction cold rolled 3% silicon strip, andapparatus such as distribution transformers utilizing such strip andmade in accordance with my invention can be produced at a saving .offrom about 25 to 40% over the cost of manufacture with standardpresent-day constructions built up from stamped laminations; and while asaving of the order of 20% may be made in small transformers such asradio transformers for standard frequencies using such strip, it ispossible by my invention to meet any given efliciency at any fluxdensity using a given quality core material 'with less such materialthan required by any other construction with which I am familiar.

The high reduction cold rolled magnetic strip which I prefer to use hasbeen available for several years. By the high reduction cold rollingprocess, striking improvements were achieved in the permeability ofnickel-iron alloys and silicon steels. This process is disclosed, forexample, in the patent to Smith et al. 1,915,766 of Jan. 27, 1933 (filedin Great Britain Oct. 31, 1930), and in the patents to Freeland1,932,306 I-8-9, Oct.

process is a general one applicable to nickel-iron alloys and siliconsteel and is, in brief, characterized by hot rolling to a thicknessconsiderably greater than the finished size followed by annealing and afurther reduction of about 60% by cold rolling to the finished size andthen heat treating. The quality may if desired be improved somewhat by afurther step of high reduction cold rolling and further heat treating.

In distribution and power transformers, re- (5 has a silicon content ofabout 3%.

actors, or the like, built in accordance with my invention, the greatestadvantage is secured at the present time by using strip material havingthe characteristics of the high reduction cold rolled 3% silicon steelinstead of the customary hot rolled material. This material can now bebought from the steel mill and while its price is at present somewhathigher than the price of ordinary hot rolled sheet material, thereductions in cost of manufacture to which I have referred can besecured in accordance with my invention even at the higher price of thematerial for the reasons which I shall hereinafter point out. Such salesof the material as have been made have, as far as I am aware, beenprincipally to motor manufacturers, such as fan motor manufacturers, whohave found it worth while to pay a little more for the material byreason of the fact that it comes in long strip and thereby makespossible a small saving in the cost of their finished punchings. It. mayhave been used to some extent in transformer cores built up oflaminations punched from the strip, but I know of no such commercial useand, for reasons upon which I have touched and shall point out more indetail, its use for such purpose does not for the standard types oftransformer constructions enable sufiicient advantage to be taken of thequalities of the material to make its use worth while. As far as I knowthere has never been any commercial use made of strip material of anyquality for winding cores of transformers or reactors for any purpose inwhich form wound conductive coils were used. There has been some use ofwound magnetic cores for instrument transformers, but not with formwound conductive windings. In all such instrument transformers withwhich I am familiar thev conductors have been wound on the magnetic coreby threading the conductors through the opening in the core, and even insuch transformers I know of no case in which the-core has beenconstructed and used so as to have the magnetic material in the properphysical condition to secure minimum watt losses as in the case of myinvention; and all instrument transformers are characterized by the useof low flux densities to permit of operation on the straight portionwell below the knee of the magnetization curve of the material so thatthe response of the measuring instrument supplied by the transformershall be in accordance with changes in the quantity to be measured andnot subject to error due to the magnetic material of the transformerbecomingmore or less saturated.

In accordance with my invention, I eliminate entirely the step ofpunching a sheet or strip into laminations. In accordance with myinvention, I use form wound coils for the electrical windings oftransformers, reactors, and the like. and wind the magnetic stripflatwise upon the coils, following a procedure which is simple andinexpensive, which reduces air gap reluctance to a minimum, utilizes thefull advantage of the favorable magnetic orientation of thegrain of thematerial, and in which in the completed apparatus the losses are aminimum due to the fact that .stresses in the .material which mightincrease its watt losses are substantially eliminated.

The high reduction, cold rolled silicon strip with which I at presentobtain optimum results Silicon steel having such a content of silicon isreadily rolled by the high reduction, cold rolled process. Somewhathigher' percentages may be used, but so far I have found no particularadvantage in increasing the silicon content although there is a slightreduction in eddy current losses by reason of the increased resistivity.The high reduction, cold rolled silicon sheet has a very pronouncedmagnetic orientation of the grain in the direction of rolling. Insteadof there being a comparatively small difference in the permeability andlosses with the grain compared with that at right angles to the grain,as in the hot rolled sheet material, the difference is very substantial.If, the losses are 100 with the grain they will be from 140 to l60 atright angles to the grain. This great difference has probably been oneof the reasons which has militated against commercial use of such coldrolled material in transformers built up of punched or stampedlaminations where at the corners of the built-up cores the magnetic fluxflows on the average at about 45 to the grain. If the losses are 100with the grain, the best that can be done with L-shaped punchings fromsuch cold rolled material cut at- 45 to the grain, is of the order of140.

With present hot rolled transformer sheets the thickness has beenlimited by experience to about 14 mils, but with the cold rolledmaterial a strip thinner than 10 mils can be produced without anyimpairment of hysteresis loss and with a reduction in eddy loss due tothe thinner sheet, making a substantially lower total loss than can beobtained with the hot rolled material. There is, indeed, someimprovement in the magnetic orientat on with the reduction in thicknessof the sheet. At present I prefer to use cold rolled sheets of 10 or 11mils in thickness and prefer in dirribution transformers to use fluxdensities of 13,000 to 15.000 gausses. although I may use still higherdensities. Owing to the advantage which I take of the characteristics ofthe material and the reduction which I secure in air gap reluctance, Iam able to use suchilux densities without sacrificing magnetizingcurrent characteristics of the apparatus.

For example, with a flux density of 14,500

- gausses the magnetizing force required for the high reduction coldrolled 3% silicon material, in the most favorable direction of thegrain, is about 3 oersteds (gilberts per centimeter), while for the best4 hot rolled material it is about 15, or five times as great. Amagnetizing force of 3 oersteds instead of producing a flux density of14,500 gauses in such hot rolled material would produce only about11,500 gausses.

For a flux density of 16,000 gausses. only about 7 oersteds are requiredfor the cold rolled 3% silicon material, while about 100 oersteds arerequired for the hot rolled l material, about thirteen times as great.

In a conventional design of transformer built up from punchedlaminations of the hot rolled l material, the magnetizing currentrequired for operation at a flux density of the order of 16,000 gaussesbecomes practically prohibitive. The magnetizing current required ismade up of two components, one to produce the flux in the material andthe other to force the flux through the air gaps in the built-upstructure, both of which components are large. In accordance with myinvention, however, operation at such high flux densities is entirelypractical with material such as the 3% cold rolled silicon steel, andgreatly improved operation from the standpoint of the requiredmagnetizing current may be secured even with the hot rolled 4 /2 siliconsteel because of the enormous reduction in air gap reluctance. Ifinstead of the standard hot rolled 4 silicon steel the G silicon steelof the Ruder application, heretofore referred to, were used, thecomparison would be still more favorable: that is to say, the 3% highreduction cold rolled material would show a still more strikingadvantage at these high densities.

My invention in its broader aspects is not limited to the use of highflux densities, however, since many of its advantages may be realizedwhere high flux densities may not be required or desired. The 5 to 6 /2%hot rolled silicon strip of the Ruder application has high permeabilityand very low watt losses at low and moderate flux densities, and it ispossible to apply such a strip by my winding process although such. ahigh silicon strip is very brittle. By heating the material and handlingit very carefully during its application I have made such coressuccessfully. However, this material is so diflicult to wind especiallyinto small diameters, that its commercial use for wound cores willprobably have to await improvement which will somewhat reduce itsbrittleness. An increased silicon content is desirable where arelatively high resistivity is desired to reduce eddy current losses andhot rolled strip having less than 5% silicon content can be very readilyapplied by my process.

The high reduction cold rolled 3% silicon steel strip has thecharacteristic which is common to all magnetic materials that elasticstrains in the material will impairits magnetic qualities. Moreover, ifthe strip is strained beyond its elastic limit; its magnetic qualitieswill be permanently impaired and can be restored only by further heattreatment. In accordance with my invention, such cold rolled 3% siliconstrip can be applied easily and rapidly without straining the stripbeyond its elastic limit and, in the completed magnetic apparatus madein accordance with my invention, the strip is free from such elasticstrains as might decrease its permeability or increase its watt losses.

In accordance with my process, I have wound, heat treated andsuccessfully applied not only cold rolled silicon steel strip having asilicon content of the order of 3%, but I have also used hot rolledmaterial having various silicon contents. For example, I have taken hotrolled silicon sheet, cut it into strip, welded the strips end-to-end toproduce along strip, and then coiled such strip into a hollow cylinderwith the turns nesting tightly within one another, heat treated the coilof strip and successfully applied it to a transformer having form woundconductive windings. The completed spirally wound magnetic core was ineach case of the same size as the heat treated coil of strip with theturns tightly engaging each other in the same sequence. I have done thiswith hot rolled sheet material having a silicon content of 2 /4% to 3%,4 4% to 5%, and 6% to 6 without in any case bending the material beyondits elastic limit, and tests of the completed transformers have shownthe absence of strains impairing the magnetic qualities of the material.

High reduction cold rolled nickel-iron alloy strip is characterized by ahigh permeability, as pointed out in the patent to Smith et al.1,915,766, heretofore referred to, but this high permeability isobtainable only at low magnetic densities. This nickel-iron alloymaterial is extremely. dimcult to handle without impairing its magneticcharacteristics. Merely dropping a piece of the material so as to giveit a sudden jar is very likely to impair its permeabiiity and increaseits watt losses so that in handling stampings of this material it hasbeen customary to pick them up and lay them down carefully: The materialis very soft and very easily bent or stretched, and it takes but a veryslight distortion after heat treatment to impair its magneticcharacteristics seriously. One of the best known of the nickelironalloys for electrical purposes is sold under the trade name Mumetalwhich contains about 74% nickel, 20% iron, 4% copper, 1 chromium, and asmall amount of manganese and silicon. When a tightly coiled hollowcylinder of this nickel-iron alloy material isheat treated, there is avery severe adhesion between the turns. I have wound up a coil of suchmaterial into a core with an inside diameter of 4 inches and an outsidediameter of 6% inches of a strip 3 inches wide and 14 mils thick. Thiscore comprised about turns and in winding the strip in order to insulatethe turns from each other I applied a thin alkyd resin varnish in whichmagnesium oxide powder has been ground in a ball mill. After winding,the core was heat treated at 1100 C. in pure dry hydrogen. The wattlosses of the core after the heat treatment were very high, indicatingexcessive adhesions between turns. I carefully separated the turns fromeach other by using a hack-saw blade which had been narrowed down toless than A; inch and the narrowed portion ground to a knife edge onboth the front and back sides and working this small blade around thespiral. After the adhesions had thus been removed the coil was tested.At a flux density of 3,100 gausses the exciting current required was.0405 ampere and the watt losses were .3897 watt. I then applied thestrip to a transformer following out the process of my invention withthe greatest possible care. After application of the core to thetransformer a test showed that an exciting current of .0810 ampere wasrequired for a flux density of 3,100 gausses. That is, the permeabilityof the strip had been decreased so that the magnetizing current requiredhad increased 100%. The test also showed that the watt losses hadincreased from the former value of .3897 watt to .5832 watt, or 50%. Inother words the magnetic permeability of the material had been decreasedby one-half and the watt losses increased by one-half due to distortionsunavoidably introduced by the process of application. For a flux densityof 6,200 gausses, the exciting current was increased well over 400% andthe watt losses by over 60%, showing that for this higher but stillrelatively low flux density, the permeability had been decreased to'less than one-quarter of that of the strip before its application tothe conductive winding. This serious impairment of the magneticqualities of nickel-iron alloy strip caused by the operation oftransferring the coil to its position in the windows of the electricalwinding, even in accordance with my present invention, makes itundesirable to use this material, as it'now exists, for my presentpurposes except in certain special applications, such as,- for example,instrument transformers, which are to be operated at low flux densities.

As heretofore pointed out, the high reduction cold rolled 3% siliconsteel strip presents no such serious difliculties as are encounteredwith strip such as nickel-iron alloy strip, and the permeability andwatt loss characteristics of the completed transformer core are as goodas those obtained with Epstein samples of the material within the limitsof the error of measurement. In

greases "making Epstein samples, the magnetic material sufllcient tomake 10- kilograms of the strip material, and the strips are dividedinto four piles of equal weight and these four piles are assembled intoeach of the four coils of an Epstein frame, the piles of strip beingcarefully assembled into a rectangle with butt joints and being held inplace by suitable holding means. The current, in making the Epsteintest, is adjusted to give the desired flux density and the watt loss isread on a wattmeter in the same way as .watt losses are read whentesting any transformer.

In accordance with my process, the strip is first wound -flatwlse on aroll or mandrel so'that the inside diameter of the spiral coil of stripmaterlal is of the exact size it is to have in the completed apparatus.The strip is tightly coiled so that the spiral turns of the strip nestwithin each other and closely engage each other, the exact 1 amount ofmaterial desired for the completed coiled core element of thetransformer or reactor being wound upon the roll or mandrel. The coiledstrip is then bound or tack welded to prevent the turns from looseningup, and is then heat treated as a unit to remove all strains and to setthe core to size and shape. Preferably, the

width of the strip is but slightly less than the width of the window inthe form wound electric coil or winding to which the strip is to beapplied, although it will be understood that my invention is not in itsbroader aspects limited to a construction in which there is only asingle strip wound core element, but embraces constructions in whichthere are two or more such core elements wound on the same or differentlegs of the conductive winding, and in large transformers it may bedesirable to provide spaces between the tightly wound core elements forcirculation of the cooling medium between the core elements. After theheat treatment the strip is then simultaneously unwound from the heattreated coil and applied to the form wound electric winding withoutbending or straining the strip material sufficiently to impair itspermeability and watt loss characteristics in the finished product.After the strip core has been applied it is tightened'to the samediameter as it had in the heat treated coil. In accordance with myprocess,

when the winding of the strip material into the window of the form woundwinding has been completed, the spirals of the strip will be in theexact condition, with the turns closely engaging each other and in thesame sequence, that they were when the coiled'strip was heat treated,and the inside and outside diameters will be unchanged. This is ofimportance if the best. results are to be secured. I have found, forexample, from tests of several particular designs of transformers'havingcores of about 4 inches inside diameter that when the inside diameter ofthe core after application to the transformer was but one-sixteenth ofan inch larger than it was when the coil of strip was heat treated, thewatt loss (at 14,000 gausses and 60 cycles) was increased from 5 to andthe exciting current about above the values obtained when the cores wereapplied with the exact diameter they had when heat treated.

If a high value of magnetic flux is to be used in the apparatus and highefficiencies with low magnetizingcurrents are to be secured, the mag-These strips are asnetic strip core material should substantially fillthe window in the insulated form wound conductive coil and the insideturn of the magnetic spiral should closely embrace the leg of'theinsulated conductive coil to which it is applied in order to provide ahigh space factor, or ratio of net copper section to the cross sectionof the space available for the winding. The window of the form woundconductive coil maybe substantially rows in the longitudinal directionof the strip and entirely with the grain. No such difliculty is presentas occurs at the corners of the conventional core construction built upof punched laminations. Air gap reluctance is reduced to a minimumbecause the turns of the spirally wound magnetic strip closely engageeach other, and the area of the air gap between adjacent turns is verylarge, being the product of the width of the strip by the length of oneturn. The length of the magnetic circuit is somewhat reduced, and airgap reluctance is enormously reducedfrom the values inherent in theconventional construction in which the coreis built up of punchedlaminations.

I am able greatly to reduce the amount of magnetic material as well asthe amount of copper over'that required in conventional designs. Forexample, I have built distribution transformers from 1 to 5 kw'.,according to my invention, with approximately one-half of the amount ofmagnetic material and approximately threequarters the amount of thecopper of the conventional punched and laminated construction with thesame full load efilciencies and substantially the same magnetizingcurrent.

The following tabulation shows, for example,

results I have obtained in three samples of distribution transformers ascompared with corresponding sizes of conventional transformers ofstandard construction:

5 kva.

My method of assembling the magnetic core on the conductive coils orwindings permits form wound coils to be used with the advantages of thesimplicity in structure, low cost of manufacture and the reliabilityincident to form wound coils. Such coils are capable of being wound soas to provide a high space factor. By using a primary coil or coilshaving a width nearly equal to the inside diameter of the spirally woundmagnetic core, and placing two secondary windings of narrower width, oneinside and one outside or one on either side of the primary winding orwindings, as hereinafter described, so as to give a stepped or cruciformsection to the winding structure, I have provided a construction fortransformers which has a relatively high space factor, and which lendsitself to conventional methods of manufacture, insulation andimpregnation.

My method of applying the coiled strip to the should be.

form wound conductive coil is particularly desirable where thin magneticmaterial is to be used, although it is applicable with many advantagesto strip 25 mils in thickness or even thicker strip. A given amount ofmagnetic material in the form of a very thin strip requires a largernumber of turns in the magnetic core than does the same amount ofmaterial with a thicker strip, but by my method of winding, the magneticstrip may be so rapidly applied that the increase in the number of turnsto be applied causes little difference in the cost of manufacture,whereas if such thin strip were to be used with the conventionalpunchings it would be difiicult to heat treat and handle them withoutintroducing too great strains in them from serious distortion, and theadditional number of pieces to be handled would considerably increasethe cost of manufacture. The punching operation introduces strains inall magnetic materials which strains must beremoved by heat treating.When the laminations are very thin they tend to stick together and arevery apt to be seriously distorted when separating them after the heattreating operation. By my process, the heat treating operation iscarried out as simply with thin strip as with-thick strip, and I havefound that the unwinding of the heat treated magnetic coil, and therewinding it upon the conductive winding actually reduces the losses ofthe magnetic coil in the completed transformer compared to the testedlossesof the heat treated coil before it has been unwound. This isprobably in large part due to the fact that during the heat treatingprocess, the turns stick together more or less at various points andcause short circuiting paths for eddy currents. These adhesions arepulled apart in the unwinding process. It is evident that the completedcore is substantially free from strains which would increase lossesbecause tests I have made show that the losses are as good as, and insome cases even better than, those obtained on small Epstein sampleswhich are free from strain.

While I have discussed the application of my invent.on to distributionand other transformers for standard commercial frequencies, it will beapparent to those skilled in the art that it is also applicable tofrequencies much higher than standard commercial frequencies. In highfrequency transformers and the like using magnetic cores, it isimportant to use very thin strip material to avoid excessive eddycurrent losses. The higher the frequency, the thinner the material If,for example, for frequencies from several hundred to several thousandcycles per second a strip of the order of 7 mils in thickness should bedesired, such thickness of strip can be very easily used in accordancewith my invention. I apprehend no difiiculty in using strip much thinnerthan 7 mils of exceedingly thin strip is desired. Low magnetic densitiesare desirable for high frequency applications and in such work it may bedesirable to use strip material having a higher resistivity than that of3% silicon cold rolled strip.

From the foregoing discussion it is believed to be apparent that myinvention involves a radical change in the transformer art where fortwenty years or more it' has been exceedingly worth while to adoptimprovements which have made even small gains. Briefly summarizing theobjects and advantages of my invention as applied to distributiontransformers, the transformer has a high all-day efiiciency based onpresent loading practice, the total losses being either equal to or lessthan those of the present design, resulting in a lower operating cost tothe public utilities. The transformers can be made smaller at veryappreciable savings, resulting in lower initial investment, and greaterreturn on the investment to the public utilities. The reduction in theweight of the transformer makes it easier to handle in the warehousesand in the field and reduces transportation costs. Better productionmethods at lower cost are available to the manufacturer and a moreuniform product can be made.

My invention will be better understood when considered in connectionwith the following detailed description and accompanying drawings.

In the drawings, Fig. 1 shows a conductive form wound winding structuresuch as the winding of a reactor to one leg of which a magnetic core isto be applied; Fig. 2 shows a cylindrical coil of magnetic stripsuitable for application to the winding structure shown in Fig. 1; Figs.3 to 7 inclusive show various stages in the method of applying themagnetic core to the winding structure; Fig. 8 shows the magnetic corecompletely applied; Fig. 9 shows the first stage in the application of amagnetic core to one leg of a form wound winding structure of atransformer and illustrates one arrangement of the primary and secondarytransformer windings; Fig. 10 is a similar view showing a modificationof the arrangement of the primary and secondary windings; Fig. 11 is afragmentary view in perspective illustrating one phase of the operationof applying the magnetic core to a transformer winding structure; Fig.12 is a view of a similar transformer winding structure with twomagnetic cores completely assembled thereon; Fig. 13 is a view inperspective showing suitable mounting means applied to a transformersuch as shown in Fig. 12; Fig. 14 is a view of a transformer in whichthe magnetic core is applied to but one leg of the winding structure;Fig. 15 is a view in perspective with parts broken away showing amachine for automatically applying magnetic cores to a transformer; Fig.16 is a view taken generally from the left of Fig. 15 indicating meansfor raising and lowering the means for holding the transformer duringthe core winding operation; Fig. 17 is a perspective showing of part ofthe mechanism shown in Fig. 15; Fig, 18 is a view showing a detail ofthe machine of Fig. 15 and Fig. 19 shows another detail.

Referring to Fig. 1, a form wound conductive winding l0 having asubstantially rectangular window II is shown. The right-hand straightleg of this conductive winding is indicated as having been surrounded bycylindrical insulating means l2. Fig. 2 shows a cylinder l3 of thinmagnetic strip material spirally wound flatwise with the turns tightlyengaging each other, the end of the outside turn being secured to thenext underlying turn in any suitable manner as, for example, by beingtacked or welded in spots as indicated at H. The cylinder or coil ofmagnetic strip is represented in the condition that it comes from theheat treating oven. The inside diameter of the cylinder of magneticstrip is the same as the outside diameter of the insulating cylinder. l2to which it is to be applied.

Fig. 3shows a section of the winding structure of Fig. 1 taken on line3-3 of Fig. 1, and it shows more clearly one suitable construction ofthe insulating cylinder 12 in which the cylinder is made up of twosemi-circular insulating pieces l5 and I6 held together by a suitablewrapping of tape. The first step in applying the core of Fig. .2 to thewinding structure of Fig. l is also shown in Fig. 3. The core I3 isplaced over a suitable post or roller I1". The tack welds I4 areloosened up as, for example, by a screw driver, and the end of the stripis carried in a clockwise direction through the window 4 and broughtaround to form a' fairly large .loop I8, the end of the strip beingsuitably secured to the next underlying turn by tying or tack weldingitat I9. The loop I8 should be small enough so that the strip may beturned freely without touching the far side of the window, and it shouldbe large enough so that when all of the materialof the coil I3 is woundinto the larger loop, the loop may be turned freely without having theinside turn of strip touch the inner side of the window. Ordinarily thenumber of turns in the large loop will be about half the number of turnsin the coil I1. and, in such case, the first turn oi the loop I8 needsto clear the insulating cylinder I2 by a distance only about half thethickness of the layer of turns of the coil I3 shown in Fig. 2. The coilI3 is then rotated together with the large loop I8 which causes thestrip to be unwoundfmm the coil I3 and simultaneously rewound into theloop I8, the turns building up successively from the outside in. It willbe observed that the direction of curvature of the strip in the largeloop I8 is the same as in the coil I3.

Fig. 4 shows a stage in the operation where about half the material ofthe coil I3 has been wound into the larger loop, and Fig. 5 shows thestage in the operation where all but a fraction of a turn of the coil I3has been wound into the larger loop I8. It will be observed that byreason of the fact that the loop I8 has a larger diameter than theoutside diameter of the coil I3, the number of layers in the large loopis so much less than the number in coil I3 that the large loop may befreely rotated through the window II in the winding structure I withoutscraping or in any way damaging the insulation on the conductive windingID, or scraping or 'damaging the insulating cylinder I2. Furtherrotation of the loop I8 beyond the position shown in Fig. permits theinside turn of the strip to coil about the insulating cylinder I2 whichit does by reason of the permanent set which wasimparted to the strip bythe heat treatment. The post or roller I1 is then lifted out of the wayand the tack weld I9 broken, whereupon the strip collapses to thegeneral shape shown in Fig. '7. By reason of the permanent set impartedby the heat treatment, the coil of strip tends to collapse to the exactphysical condition it was in when heat treated, but the friction of theedge of thestrip on the table tends to cause the spiral turns to assumethe shape shown in Fig. '7. The operator will then secure the insideturn of the strip to the insulating cylinder I2 and then work the turnsof the strip around by hand to close the turns of the strip down intothe completed form I3 shown in Fig. 8. Each turn of the magnetic stripin the cylindrical coil now occupies exactly the same position it didwhen the coil I3 was heat treated. The inside and outside diam- ,etersare the same as they were in the heat treated coil, and the turns are inthe same sequence that they were in the heat treated coil.

. The inside turn closely embraces the cylinder I2.

from freely turning through the window II.

along the end of the strip of the heat treated coil and then tighteningthe turns upon the completion or the winding operation until the end ofthestrip lines up with that mark.

High reduction cold rolled silicon steel strip having a silicon contentof the order of 3% takes a definite permanent set when heat treated toremove elastic strains, and the elastic character of such strip permitsit to be very readily applied by my method oi' winding without bendingit beyond the elastic limit. While the strip is in the larger loop IIit, of course, contains strains but when the coil is flnally collapsedto its final position in the completed apparatus, such strains areabsent. During the winding operation the strip is not distorted in anyrespect so as to impair its magnetic qualities, and tests made'oi thecore in the completed apparatus show that the permeability and watt losscharacteristics are as good as those obtained from strain-tree Epsteinsamples.

The unwinding and rewinding oi. the coil of strip greatly improve thewatt loss characteristics over those 01 the coil as it comes from theheat treating oven. This is probably due to the fact that the heattreatment for at least an hour at a temperature oi! about 800 C. toabout 900 C. in the heat treating oven produces slight adhesions betweenthe turns of the strip providing short circuiting paths for eddycurrents- The unwinding operation separates these adhesions without theslightest difllculty.

The elasticity of high reduction cold rolled 3% silicon strip is suchthat the large loop ll can be considerably more elliptical than thatillustrated in Figs. 3 to 6 without straining the material of thestripbeyond the elastic limit. This quality of the strip may be availedof advantageously where the room available for the loop is limited bythe presence of several winding structures around the adjacent legs ofwhich the strip is to be wound simultaneously. For example, if there arethree such winding structures spaced 120 degrees apart with theiradjacent legs nested together so as substantially to fill the opening inthe cylindrical core when applied, it may be necessary to make the largeloop somewhat elliptical to prevent its rubbing against the outsideportions of the adjacent winding structures. The length of the loop canbe varied by adjusting the position 01 the post or roller H to vary thedistance between the post and the leg of the conductive windingstructure upon which the strip is to be wound. With more brittle stripsuch as strip containing a high percentage of silicon or with softerstrip which cannot be bent so much without exceeding the elastic limit,the loop I8 should be substantially circular and not large enough indiameter to straighten out the strip too much when its curvature changesfrom that of the original coil to the larger loop. The loop should,however, always be large enough so that the winding operation may becompleted without having the number of layers of strip in the loop I8sufliciently great to prevent the loop By reason of the fact that agreater amount of material may be accommodated in the larger loop with asmaller number of layers, it is possible in accordance with my inventionto wind enough strip through the window in the conductive winding sothat the strip is collapsed to its final form the number of layers willbe sufllcient to flll the window in the winding structure irrespectiveof whether the cross section of the leg of the winding structure uponwhich the strip is wound, and the corresponding, inside diameter of thecoil of the strip in its heat treated form, is large or small. It isthus possible, in accordance with my invention, to have the conductivewinding structure fill the opening in the cylindrical coil and have thecore fill the opening in the winding structure permitting an arrangementwhich gives the minimum length of turn of the conductive winding and theminimum length of path for the magnetic flux produced in the core by thewinding and, as heretofore pointed out, the air gap reluctance betweenturns of the strip is reduced to a minimum, and by having the mostfavorable magnetic orientation of the grain of the strip in thelongitudinal direction of the strip, the exciting current required forany given flux density is kept down by reason of the high permeabilityof the material and the efiective use of both copper and magneticmaterial. The economy in the core of magnetic material reduces the totalwatt loss.

I have referred in the foregoing description to adhesions between turnsof the magnetic strip produced in the heat treating oven. The amount ofinsulation required between turns of the strip is very small. Heattreated coils of strip will ordinarily have, without any specialprovisions for such insulation, suflicient insulation between turns tokeep the'eddy current loss from becoming appreciable if the slightadhesions between the turns referred to are separated during the windingoperation. If, however, the magnetic qualities of the strip have beenimproved by resort to special expedients such as pickling in acid inconjunction with an annealing treatment under deoxidizing or purifyingconditions, as pointed out for example in the patent to Ruder 1,648,697,Nov. 7, 1927, it may in some cases be desirable to apply an insulatingsurface to the strip. A solution such as a chrome silicate solutionwhich will survive the heat treatment can be used. However, with myprocess of winding it is a simple matter to spray on a very thininsulating coat while the core is being unwound and applied to thewinding structure in which case the insulating material does not need tobe of a character to survive the heat treatment.

The application of wound strip magnetic cores to transformers may becarried out in accordance with my invention just as simply as theapplication of such cores to reactors. The form wound winding structureIn shown in Fig. 1 may indeed represent either the conductive winding orwindings of a reactor or the conductive windings of a transformer. InFig. 9 I have shown the first step in the application of such a core toa transformer having a conductive winding structure comprising a primarywinding 20 and two secondary windings 2-1 and 22, the winding 2| lyingoutside of the winding 20 and the winding 22 lying inside of the winding20. The arrangement of these windings more clearly appears from thefragmentary perspective view shown in Fig. 11. The windings 20, 2!,and22 are form wound coils which may be wound, insulated, and, ifdesired, impregnated, simply, effectively, and inexpensively.

Referring further to Fig. 9, the first stepof applying the coil ofmagnetic strip 23 to the conductive winding structure of the transformeris illustrated, the stage of the winding operation corresponding to thestage shown in Fig. 3 of applying such a coil of strip to a reactor.-The end of the coil 23 has been brought through the window 24, a largeIoop 25 has been formed and the end of the strip has been secured as bytack welding at 28 to the next underlying turn of the coil 23. This tackwelding may be done in various ways, one simple way being to press acarbon; electrode of a welding circuit momentarily against. the strip,preferably holding a small piece of wood or the like under the strip towhich the end is to be tacked to facilitate the operation,'the otherterminal of the welding circuit leading to the coil of strip. The coil23 and large loop 25 are then. rotated in the direction shown by thearrow to unwind the coil 23 and rewind the strip into the inside of thelarger loop 25 with the large loop passing freely through the window 24.This operation may be carried out manually, as heretofore described inconnection with Figs. 3 to 8 inclusive. It may be also carried outautomatically, and Fig. 9 illustrates how this may be simply and rapidlydone. The roll 21 corresponds to the post or roller ll of Fig. 3, and asimilar roll 28 is mounted against the outside surface of the coil 23.By driving the rolls 21 and 28 simultaneously in opposite directions asindicated by the arrows while the roll 21 is pressed toward the roll 28by a suitable spring mechanism which keeps the rolls 21 ,and 28 inengagement with the inside and outside surfaces of the coil 23, the coilmay be unwound forming the loop 25 rests upon a suitable table,

not shown in Fig. 9. To prevent any tendency of the strip to climb upthe rollers. 21 and 28 during the winding operation, a roller 3| may beprovided which bears on the top edge of the coil of strip 23 and the topedge of the loop of strip 25.

When the coil of strip 23 has all been unwound into the larger loop 25and the inside end of the coil of strip 23 has curved around theinsulating cylinder 32, as heretofore described'in connection with Figs.5 and 6, the roller 21 is removed and the tack weld 26 broken, whereuponthe strip of the large loop 25 assumes the general shape shown in Fig.7. The inside end of the strip is then clamped to the insulatingcylinder 32 as, for example, by the clamp 33 shown in Fig. 11. The stripis then collapsed .to the completed form, as heretofore described inconnection with Figs. 7 and 8, and the outside end of thestrip issecured to the next underlying layer of strip in any suitable way, asfor example, by tack welding. The clamp 33 is then removed. Theconductive winding structure may then be turned around and a similarcoil of strip wound on the other leg of the winding structure. Fig. 9represents a stage of the operation in which a cylindrical coil of strip34 has already been applied to the other winding leg. It will beobserved from Figs. 9 and 11 that the conductive winding structure has acruciform cross section, the primary winding 20 extending diametricallyof the opening in the hollow cylindrical magnetic core while thesecondary windings 2| and 22 are narrower than the primary winding 20andoccupy the spaces left in the opening in the core.

The periphery of the conductive winding structure has a step shapedperiphery which generally approximates the circular periphery of theinside of the hollow core. The arrangement, in which the opening in thecore is substantially fllled by the conductive windings, provides a highspace factor or ratio of net copper cross section to the cross sectionof the space available forthe conductive wingings, 'only such spacebeing left as is requisite for insulation and circulation of the coolingmedium where spaces are desired for such circulation; My invention isnot limited to form wound coils of rectangular cross section, as shownin Fig. 11, but such coils are more simply and inexpensively wound thancoils of a cross section which would still more completely flll thespace available for the conductive windings.

Instead of having the primary and secondary windings locatedconcentrically, as shown in Figs. 9 and 11, they may be locatedside-by-side, as shown at 20', 2|, and 22', in Fig. 10. This arrangementalso provides the desired cruciform or stepped cross sectionsubstantially'fllling the opening in the strip wound cylindrical core.

Fig. 12 shows a completed transformer with the primary windings 20, 2i,and 22 arranged as in Fig. 9 and two completed strip wound cores 34 and35 applied thereto. The tack welds for the outside end of the strip ofthe core 35 are indicated at 36 in Fig. 12. The width of the magneticstrip may be substantially as great as the length of the window 24 inthe winding structure, and the number of turns passing through thewindow may be sufiicient substantially to hi] the window with magneticstrip.

While a single thin strip of the magnetic material is fairly delicateand subject to injury by distortion, the completed cylindrical coreswith the turns tightly engaging each other are rugged.

The completed transformer of Fig. 12 may be very simply provided withmounting means. Fig. 13 shows one form of mounting applied to thetransformer of Fig. 12. The mounting shown comprises a pair ofchannel-shaped members 38 and 39 secured together by strap members 40,M, 42. and 43. These strap members may be welded to the channel member38 and bolted to the channel member 39, as indicated by the'bolts 44,suitable spacers I! being provided. The upper ends of the strap members40, ll, 42, and 43 may be provided with lugs and bolt holes for securingthe transformer in a tank filled with insulating fluid. Intheconstruction shown in Fig.

13, the strap members 40 and 43 are shown as provided with bent-overlugs 46 and 41 provided with oblong holes for securing them in place,and the strap members 4| and 42 are indicated as straight withsuitablecircular holes, but it will be understood that the particulararrangement used may be changed as desired.

The transformer of Figs. 12 and 13 is preferably mounted horizontally asshown in Fig. 13 so that the insulating fluid may circulate freelythrough the spaces between the windings. The channel members 38 and 33are provided with openings permitting a free circulation of theinsulating fluid, two of such. openings being indicated in the channelmember 38 at 48 and. Suitable terminal mounting means may be provided.In Fig. 13, an insulating member I is shown forv mounting the highvoltage terminals II and I2 of'the primary winding. An insulating member'3 is shown for leading out the con-.

ductors I connected to the-secondary windings.

The secondary windings may be connected in series or parallel, as may bedesired.

While I have shown the secondary winding 2| surrounding the primarywinding 20 with the winding 20, in turn, surrounding the secondarywinding 22 in Figs. 9 and 11 and have shown the secondary windings 2iand 22 located one each side of the primary winding 20 in Fig. 10, itwill be understood that my invention in its broader aspects is notlimited to any particular arrangement of such windings or to the numberofsuch windings. In some cases, it may be desirable further to divide upthe windings. For example, the primary winding 20 may be divided intotwo windings with a space between to facilitate circulation of thecooling medium, and such an arrangement of windings is shown in Fig.hereinafter described. Where the secondary windings are locatedalongside the primary winding, as shown in Fig. 10, the best circulationof the cooling medium will ordinarily be attained where the transformeris mounted vertically instead of horizontally as in Fig. 13.

Instead of dividing the magnetic core of the transformer into twocylinders, one wound rm each leg of the transformerras shown in Fig. 12,a single magnetic core wound on only one leg may be provided. Such anarrangement is shown in Fig. 14, wherein the core 55 is mounted on oneleg of the winding structure, as in the case of the reactor.- shown inFig. 1. It will be understood, however, that my invention is not limitedto a construction in which the magnetic strip is applied to only one ortwo of the legs of a conductive winding structure.

In Fig. 15, I have shown one form of machine which I have found veryeffective for applying strip-wound magnetic cores to winding structures.This machine is disclosed and claimed in my application Serial No.123,250, filed concurrently herewith, but it is shown and described herein order to show how the transformer and reactors embodying my inventioncan be made by carrying out the process of my invention automatically.

Fig. 15 shows a transformer comprising two primary windings 50 and twosecondary windings 51 and 58 located respectively outside and inside ofthe primary windings to provide winding legs having a cruciformcross-section, as heretofore described. The completed transformer isshown having two strip-wound magnetic cores 59 and 60 applied thereto.Fla 15 shows that phase in the operation of manufacturing thetransformer at which the transformer has been completed and is ready tobe lifted out of the machine preparatory to inserting another windingstructure to which magnetic cores are to be applied.

The winding structure is clamped in a winding head, In the arrangementillustrated, the winding structure is shown clamped between a lower setof four rollers 6i and four upper rollers 62, which rollers arepreferably provided with surfaces of rubber or the like so as not todamage the transformer windings when clamped against them. The rollers6i are mounted on a member 63 near the bottom of the winding head andthe upper rollers 81 are mounted 'on a member 64 near the top of thewinding headi. Both of the members 6 3 and 64 are mounted so that theymay been wound on one leg of the winding structure,

the winding structure may be turned through 180 degrees to put the otherwinding leg in position to have the core applied thereto. The windinghead comprises a back member 65 having a lower forwardly projectingmember 66. for supporting the pivoted member 63 and an upper forwardlyprojecting member 81 through which a threaded rod 88 operated by ahand-wheel 69 projects for raising and lowering the member 64 pivoted atthe lower end of the rod 88. By turning the hand-wheel 69, the member 64may be moved downwardly to cause the winding structure to be clampedbetween the lower rollers 6| and the upper rollers 62. A rod I0 biaseddownwardly by a spring or gravity is provided with a. projection adaptedto fit into openings II, one of which openings is provided in each endof the member 04. When the Winding structure has been clamped betweenthe rollers 6| and 62, it is turned about its axis until the projectionon the rod I0 fits into one of the openings "II in the member 64, thusholding the winding structure in the position shown in the drawing.Upon'raising the rod 10 slightly, the winding structure can be turnedthrough 180 degrees, whereupon the projection on the rod 10 will enterthe opening 'II in the other end of the member 84 and hold the windingstructure in its new position.

To remove the transformer from the winding head, it is merely necessaryto turn the hand wheel 69 to back off the threaded rod 68 and raise theupper clamping member 64, whereupon the transformer can be lifted outand a new winding structure put in and clamped in place by lowering theclamping member 64. v

The winding structure should then be lowered into the table I2 so thatthe top surface of the table will be even with the point on the windingstructure where the bottom of the cylindrical magnetic core to beapplied will come.

To permit of raising and lowering the winding head bodily with thetransformer clamped therein, the member 65 is arranged to be movedvertically on the standard I3, as shown more clearly in Fig. 16. Inorder to guide the member 65 during its movement, rollers I4 and I5,mounted on themember 65, engage the side edges of the standard 13, androllers 16 mounted on the forwardly projecting member 81 of the member65 bear on the back of the standard I3. To raise and lower the windinghead, means is provided comprising a rack 11 welded to the edge of theplate 65, a pinion I8 engaging the rack and a hand wheel I9 foroperating the pinion. The shaft of the pinion 18 and hand wheel 19 ismounted in a bearing on a supporting member 80.

The member is secured to a member I58 hereinafter referred to which issecured to the table 12 but adapted to be adjusted to move the windinghead longitudinally of the table as hereinafter described. To hold themember 65 in any position to which it has been adjusted vertically, adog 8i is arranged to engage the rack TI, and a clamping screw 82 isprovided passing through a slot 83 in the standard 13 to clamp themembers- 'a pulley 0.

support 81 which may be clamped in any adjusted position by the wing nut88. The roller 89 for bearing on the top edge of the strip duringwinding corresponds to the roller 3| of Figs. 9 and 10, this rollerbeing mounted on a shaft 90 which may be clamped in any adjustedposition in the member 9| by the set screw 92.

The 21 of Figs. 9 and 10 is shown in Fig. 17, and this roller isremovably mounted in a member 94 which is rotatably mounted in a memberand driven by suitable gearing, as hereinafter described. The roller 93is shown with a projecting shaft 96 and a collar 91 provided with across pin 98 which fits in slots 99 in the member 94 so that when theroller is placed in position in the member 94 it is rotated by themember 94 through the pin and slot connection.

Assuming that the winding head has been lowered to bring the windingstructure into proper position with reference to the table I2 forapplicaroller 93 which corresponds to the roller I tion of the magneticstrip core, the coil of strip, as

it comes from the heat treating oven as shown in Fig. 2 for example, isplaced on the table I2 and the roller 93 put into place inside the coilso that the rollers 93 and 84 occupy the positions of the rollers 21 and28 shown in Figs. 9 and 10. In order to bias the roller 93 toward theroller 84 and clamp the coil of strip between these two rollers, themember 95 in which the member 94 is rotatably mounted is supported onrods I00 and IOI sliding through openings which constitute hearings inthe table I2. The outer ends of the rods I00, IOI are bolted to a crosspiece I02 which slides on guide pin I03 supported in the frame I 04which is fixed to the table I2. Springs I05 and I06 bearing against thetable I2 and the cross piece I02 bias the roller 93 toward the roller84.

Preliminary to placing the coil of magnetic strip on the table, theoperator moves the member 95 against the bias of the springs I05 and I06by lifting up on the lever I01, which liftin movement moves the crankI08 downwardly the end of this crank being secured to the cross pieceI02 by a flexible rope or chain I09 passing over The pawl II I engagingteeth on the member II2 mounted on the shaft carrying the lever I01 andcrank I08 holds the cross piece 95 against the bias of the springs.Theroller 93 may then be readily mounted in the member 94, and the coilof magnetic strip placed on the table over the roller 93. The operatorthen lifts up slightly on the lever I01 and releases the pawl I II andpermits the springs I05 and I08 to press the roller 93 against theinside of the coil of magnetic strip, the outside of which is pressedagainst the roller 84. The end of the coil of magnetic strip is thenloosened up and carried through the window of the' winding structure andbrought around in the relatively large loop and tack welded to the nextunderlying turn 01' the coil of magnetic strip, as described inconnection with Fig. 3 and also in connection with Figs. 9 and 10.

As heretofore described, the rollers 88 and 84 are driven to unwind thestrip from the heat treated coil and rewind it into the larger loop,winding from the outside in as described in connection with Figs. 3 to6. To drive the rollers 98 and 84, amotor II3 is provided operatingthrough'suitable pulleys and a belt H4 to drive the shaft II5 shown inFigs. 15 and 17. As shown in Fig. 15, the drive is through beveled gearsII! 76 and a shifting clutch IIl controlled by a foot lever "I I0.

As shown more clearly in Fig. 1'7, a gear H0 is secured to the upper endof shaft H0 and this gear drives a pinion I20 which turns the roller 04.The pinion I20 also drives the gears I2I and I22 which in turn drive thepinion I20 which turns the member 94 and roller 90. The rollers 04 and00 turn in opposite directions. The shaft of the gear I2I is fixed, butthe gear I22 is floatingly mounted between members pivoted about theaxes of the gear I2I and pinion I20, the two upper members being shownat I24 and I20. These members or links, two of which are shown at I24and I20, are pivoted to each other and support the gear I22. Thisarrangement permits the gearing to remain in mesh notwithstanding thehorizontal movement of the member 90 heretofore described.

In order to permit the winding structure of the transformer to belowered into the table 12 and raised-therefrom, an opening is providedin the table, and to prevent any difficulty from the coil of strip orany part of it going into this opening, a removable member I20 may beset into the opening with its top surface level with the top of thetable 12. The position of this member I20 in the opening may be adjustedand its proper position in the opening may be determined by the positionof a 'stud screwed into the member I20 and locked in the desiredposition by the nut The step of collapsing the turns from the positionshown in Fig. '1 to the position shown in Fig. 8 has been heretoforedescribed as carried out manually. In Fig. 15, however, I show mechanismfor carrying out this step automatically which I have found operatesvery successfully.

This is accomplished in the machine shown in Fig. 15 by rotating rollersI29 and I29 havingfriction surfaces of rubber or the like, which rollersare moved manually to bear against the sides of the large loop when itis in the stage of operation shown in Fig. 7. The rotation of theserollers rapidly collapses the loop and as the size of the loopdecreases, the rollers are moved inwardly until the coil assumes itscompleted shape, after which the rollers are moved out of the way andthe outer end of the magnetic strip tackwelded in place, as heretoforedescribed. The rollers I20 and I29 are respectively mounted at the endsof pivoted supporting arms I00 and IOI, the pivot for the arm I00 beingshown at I02. To drive the rollers I20 and I29, belts I00 and I04 areprovided opera ng on suitable pulleys. The pulleys at the pivo (1 endsof the arms I00 and IOI are mounted on vertical shafts, one of which isshown at I00, and driven through bevel gears, one set of which is shownat I04. These bevel gears are located at theends of shafts I00 and I00.The shaft 100 is driven through bevel gears I01 from the shaft.i00 whichis operated by a bevel pinion I09 engaging the bevel gear I40 on theshaft IIO, shown in Fig. 17. The-shaft I00, also shown in Fig. 17, isdriven by the bevel gear I40 through the bevel pinion I. The rollers I20and I29 are driven with the same direction of r'oitation. Inconsequence, the points of tangency th the coil of magnetic strip haveopposite directions of linear motion and cause the layers of magneticstrip to be wrapped around the winding-structure in much the same manneras occurs when an operator twists the coil manually by the aid of thefriction between his hands and 10 theouterlayergfthecoil.Theeflectistoclose does not ?ppear in Fig. 15, turns the member I44about ts pivot I40. One end of the member I44 is connected through meansI40, which will be more clearly described hereinafter in connection withFig. 19, to a member I" rigidly secured to the supporting arm for theroller I20 so that upon moving the lever I42 and turning the member I44,the member I40 moves the roller I20 about the pivot I02. A member I40,like the member I40, moves the support for the roller I29 about itspivot. It will be apparent that the rollers I29 and I20 may be movedtoward each other to close up the loop of strip while still being drivenrotatably. In order to ensure that the rollers I20 and I20 engage theloop gently and to assist the operator in keeping the rollers inengagement with the loop while it is being collapsed, the members I40and I40 are constructed as shown in Fig. 19. The member I40 for examplecomprises an outer cylinder I40 and. a plunger I40. The plunger I40 ispivotally secured to the member I44 and the cylinder I40 is pivotallysecured to the member I41, which in turn is attached rigidly to theunder portion (not visible in the drawing) of the roller-supporting armI00. A spring I00 located in the cylinder provides a yielding pressureon the roller I20 on the loop of strip. In order to limit the movementof the plunger I49 in the cylinder I40, a slot IOI is provided inthecylinder and a pin I02 engaging the slot extends through the plungerI49. The result is a kind of springpr'essed yielding dash-pot connectionin which the movement of the plunger in the cylinder is limited by thelength of the slot. The member I40 likewise provides a yielding pressureon the roller I29. be used for applying the strip wound core to awinding structure in which the core is applied to either one or bothlegs of the winding structure. If it is used for applying such cores toboth legs, after one core has been applied the winding structure israised out of the opening in the table 12 by operating the hand wheel19. The rod 10 is slightly raised lifting the projection out of the holein the end of the member 04, and the winding structure is turned through180 as heretofore described, whereupon the projection on the rod I0 willengage the other opening 'II of the member 04 after which the windingstructure is moved to the lower position and the other magnetic coreapplied.

To accommodate various types and sizes of winding structures it isdesirable to have the winding head movable longitudinally of the tableto vary the distance between the leg to be wound and the roller 90. Toaccomplish this longitudinal movement a hand wheel I00 is providedtuming a pinion I04 which engages the rack I00 secured to the member I00which slides on the top of the table 12. Another member I0! is providedwhich slides on the bottom surface of the table 12. A clamping screw I00is provided which is loosened up when it is desired to adjust thelongitudinal position of the winding head and which is tightened up toclamp the table I2 between the members I00 and I01 in the adiustedposition. Since the transformer is heavy and its weight is all in front"of the plate 00.'

forwardly causing the parts I56 and I5I to bind on the table I2. Toprevent this binding which would interfere with ready longitudinalmovement, a member I59 is bolted or otherwise secured to the standard I3and member I56, which member I59 has a pair of forward projections I60,one of which is shown in Fig. 15. On the forward ends of theseprojections rollers are mounted, one of which is shown at I6I in Fig.15. These rollers bear on track members, one of which is shown at I62 inFig. 15, which are in turn mounted on the supporting plate I63 whichalso supports the table I2 and various parts of the machine heretoforedescribed.

' As I have pointed out in the foregoing description, the form woundconductive winding structures may be impregnated with suitableinsulating compound and. the magnetic cores wound thereon after suchimpregnation. If this practice is followed, it is relatively easy toremove such cores if it becomes necessary to do so. While the magneticstrip material preferably substantially fills the window in theconductive winding structure in transformers such as distributiontransformers, a space of about inch will usually be left, and this issufficient to permit loosening up the coil of magnetic stripsufficiently to permit its turning about the leg of the windingstructure to which it has been applied. It is merely necessary to loosenup the tack welds holding the outer end of the strip and loosen up theturns by pressing on the sides of the strip and imparting a turningmovement in a direction to unwind the strip. In this way, a slightclearance may be produced between the inside turn and the insulation onthe winding leg, after which the end of the inside turn is tack weldedto the next adjacent outer turn at the edge of the strip. The outsideend of the strip may then be placed around a mandrel and unwound fromthe winding structure about which the cylinderof magnetic strip turns,the tack welding of the inside turn preventing the closing up of thecoil of strip to grip the leg upon which it turns. The winding of thestrip onto the mandrel will produce a coil of strip, the turns of whichwill have a sequence opposite to that which existed in the coil of stripon the apparatus. This permanent- 1y impairs the magnetic qualities ofthe strip which must be again heat treated if its original condition isto be restored. If the impregnation of transformers, reactors, and thelike, is carried out after they have been completely wound, it is muchmore diificult to remove the magnetic strip, and after it has beenremoved it is likely to have no value above its scrap value.

While I have shown particular forms of electromagnetic inductionapparatus embodying my invention and have illustrated a particularmechanism for practicing my process, it will be apparent to thoseskilled it the art that many changes and modifications may be madewithout departing from the spirit and scope of my invention and I aim inthe appended claims to cover such changes and modifications.

I use the expression form-wound, as applied to electrical coils, todenote the type of coil which is wound on a former or winding-form,taped or otherwise insulated, and usually impregnated, and which,usually assembled with other such coils in the form of a coil assembly,is thereafter combined with a magnetic core, as distinguished, forexample, from the type of coils,

75- known as -core woundf coils, produced by winding electricalconductors on a previously assembled part (or whole) of a magnetic core.

The term unwinding as used herein does not refer to a mere progressiveseparation of the turns of the coil, such as occurs, for example, whenone passes a needle between the turns of the hair spring of a watch.Such an operation does not destroy the identity of the coil. I use theterms unwinding and rewinding to refer to true unwinding and rewindingoperations, in which during the unwinding the turns are progressivelyremoved from a coil, and in which during the rewinding the turns areformed into a new and different coil. Such an operation necessarily andinherently involves that the new coil has either an axis or a diameterdifferent from that of the original coil.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. Electromagnetic induction apparatus comprising a conductive windingof the form wound type and having a leg which is straight forsubstantially the length of the window of the conductive winding, and amagnetic core substantially filling said window and composed of thinmagnetic strip material including a length 01' strip wound flatwisespirally many times around said leg, the inside surface of said magneticcore closely embracing said leg so that the cross section of theconductive winding in said leg bears a high ratio to the cross sectionalarea of the opening in said core, the turns of said length of stripnesting tightly within one another, said strip having the most favorablemagnetic orientation of the grain of the material in the longitudinaldirection of the strip and said core being characterized by substantialfreedom from elastic strains which would impair the magneticcharacteristics of the material.

- 2. Electromagnetic induction apparatus comprising a conductive windingof the form wound type, a magnetic core for said conductive windingcomprising a cylindrical core element passing through the window of saidconductive winding and composed of thin magnetic strip material tationor the grain of the material, the spiral turns of said cylindrical coreelement tightly engaging each other to reduce air gap reluctance, thepermeability and watt loss characteristics of said completed cylindricalcore element at any given magnetic density and frequency beingsubstantially as good within the limits of error as those of strain-freeEpstein samples of the same material cut in the direction of the mostfavorable magnetic orientation of the grain of the material and testedby present standard methods of measurement of such samples.

3. Electromagnetic induction apparatus comprising a conductive windingof the form wound type, and core means comprising magnetic stripmaterial substantially filling the window in said conductive winding sothat the cross section of said core means is substantially the same asthe opening in said cylindrical core and arranged cross section of thespace available for said core means, said core means comprising spirallywound thin flat strip the spiral turns of said strip nesting within eachother and closely engaging each other to reduce air gap reluctancebetween the turns, the most favorable magnetic orientation of the grainof the material being in the longitudinal direction of the strip so thatthe magnetic flux will be in the direction of the most favorablemagnetic orientation of the grain, said material having at leastsubstantially as high permeability and substantially as low watt losscharacteristics as high reduction cold rolled 14 mil steel stripcontaining.3% silicon, and the inside turn of the spiral closelyembracing the side of the insulated conductive coil so that in thecompleted apparatus the side of the conductive winding located in theopening through which it passes in the spirally wound core substantiallyfllls such opening to provide a high space factor, the material of saidstrip in the completed apparatus being substantially free from elasticstrains, whereby for any given emclency and any given magnetiflngcurrent and at any desired flux density less magnetic material andcopper are required than for constructions of the same kva and voltagerating built up'of laminations punched from either high reduction coldrolled 3 to 3 silicon strip or hot rolled 3 to ti /2% silicon sheet andwhereby with a substantial reduction in the amount of magnetic materialand copper and at flux densities of the order of 15,000 gausses anefllciency substantially the same as that obtainable in such built-upconstructions of such cold rolled or such hot rolled material may beobtained with little or no increase in magnetizing current over thatrequired in such built-up constructions for the same kva and voltagerating at flux densities therein of the order of 12,000 to 13,000gausses.

4. Electromagnetic induction apparatus comprising a conductive windingand a magnetic core of thin magnetic strip material wound flatwisespirally into. the form of a hollow cylinder with the turns of thespiral tightly engaging each other, said core being free from adhesionsbetween turns which would provide short circuiting paths for eddycurrents, the material of said strip being characterized by a permanentset due to heat treatment, said completed core being characterlzed bythe same permeability and watt loss characteristics as wouldcharacterize a cylinder of the same strip material which had been madeby winding said strip material into a cylinder having the same size andsame sequence oi turns and heat treating it to give it a permanent setand then unwinding said cylinder and rewinding it into a cylinder of thesame size having the same sequence of turns withoutdistorting it beyondthe elastic limit, said strip material being characterized by the factthat it has an elasticity when set by heat treatment sufllcient topermit such unwinding after heat treatment into a loop of suflicientlylarger diameter than the outside diameter of said cylinder when heattreated to accommodate all of the strip material with about hali thenumber of turns nesting within each other in such larger loop withoutintroducing strains sufhcient to impair the permanent set or themagnetic qualities of the strip material, the turns in said core .r. thecompleted apparatus being in the position which they tend to assume byreason of the permanent set due to the heat treatment, the turns of saidconductive winding passing through and substantially filling. the

when excited to produce a magnetic flux in the longitudinal direction ofthe magnetic strip.

5. A transformer having a magnetic core composed of high reduction coldrolled silicon steel strip having a silicon content of about 3% and of athickness not exceeding 14 mils, said core being in the form of a hollowcylinder the length of which is equal to the width of the strip, thestrip being coiled spirally flatwise with the turns closely engagingeach other, and a winding structure comprising primary and secondarywindings passing through the opening in said cylinder and substantiallyfilling the same to produce a magnetic flux flowing longitudinally ofsaidstrip, the most favorable magnetic orientation of the grain of saidstrip being in the direction of the length of the strip, thepermeability and watt loss characteristics of said core beingsubstantially the same as in a cylindrical core of the same stripmaterial which has been wound tightly to the same inside and outsidediameter and heat treated to eliminate elastic strains and then unwoundand rewound into a cylinder of the same inside and outside diameter withthe turns in the same sequence without introducing strains sufllcient toimpair either the permeability or the watt loss characteristics of thematerial.

6. An electromagnetic induction apparatus comprising a conductivewinding structure of the form-wound type and a strip of magneticmaterial passing flatwise in closely superposed turns many times aroundand closely embracing and interlinking said winding structure, saidstrip having the most favorable magnetic orientation of its grainsubstantially in the longitudinal direction of the strip, and beingcharacterized by substantial freedom from adhesions between turns tominimize eddy current loss, and by substantial freedom from eflects ofstrain beyond the elastic limit, and also from elastic strain sufllcientsubstantiallyto impair the magnetic characteristics of the material.

'7. An electromagnetic induction apparatus having a ccnductive'windingstructure and a core element consisting of 'a coil of thinhighreduction, cold-rolled magnetic strip free from strains below theelastic limit, wound many times flatwise around and closely embracingand interlinking the winding structure and having the most favorablemagnetic orientation of its grain substantially in the direction inwhich it is wound, the magnetic qualities of which strip have beenimproved by heat treatment prior to the winding operation, and which.has been wound in place after the last heat treatment in such manner asnot to produce such bending or distortion as substantially to injure itsmagnetic qualities by strains above the elastic limit.

8. A transformer having a magnetic core element composed ofhigh-reduction cold-rolled silicon steel strip having a silicon contentof about, 3% and of a thickness not exceeding about 14 mils, said coreelement being in the form of a hollow cylinder the length of which isequal to the width of the strip, the strip being coiled spirallyflatwise with the turns radially supersaid strip being in the directionof the length of the strip, the permeability and watt losscharacterlstics of said core element being substantially the same as ina cylindrical core element of the same strip material which has beenwound tightly to the same inside and outside diameter and heat treatedto eliminate elastic strains and then unwound and rewound into acylinder of the same inside and outside diameter with the turns in thesame sequence without introducing strains sufficient to impair thepermeability and watt loss characteristics of the material.

9. Electromagnetic induction apparatus comprising, a magnetic core ofthin magnetic strip material wound flatwise spirally into the form of ahollow cylinder with the turns radially superposed and a conductivewinding having turns passing through the opening in said core andarranged, when excited, to produce a magnetic flux in the longitudinaldirection of the magnetic strip, the material of said strip beingcharacterized by a permanent set due to heat treatment and having itsmost favorable magnetic properties with a direction of magnetic fluxalong the length of the strip, said core being substantially free ofadhesions between turns which would provide short-circuiting paths foreddy currents and being substantially strain free, having its turnsoccupying the positions which they tend to assume by virtue of thepermanent set of the material due to said heat treatment.

10. Electromagnetic induction apparatus comprising, a magnetic core ofthin magnetic strip material wound flatwise spirally into the form of ahollow cylinder with the turns closely engaging each other and aconductive winding having turns passing through the opening in said coreand arranged, when excited, to produce a magnetic flux in thelongitudinal direction of the magnetic strip, said strip being composedof high-reduction cold-rolled silicon steel characterized by a permanentset due to heat treat ment and having its most favorable magneticproperties in the longitudinal direction of the strip, said core beingsubstantially free of elastic strain and having its turns occupying thepositions which they tend to assume by virtue of the permanent set ofthe material.

11. The method of producing an assembled magnetic core and winding, saidmethod including the steps of winding a strip of magnetic sheet materialinto a coil, passing the outer end of said coil through the winding andsecuring it to the outer surface of the coil to form a loop around oneside of the winding, and rotating the coil to unwind it andsimultaneously rewind it around one side of the winding.

12. The method of producing an assembled magnetic core and winding, saidmethod including the steps of winding a strip of magnetic sheet mate ialinto a coil, passing the outer end of said coil through the winding andsecuring it to the outer surface of the coil to form a loop around oneside of the winding, rotating the coil to unwind it and simultaneouslyrewind it around one side of the winding, detaching the outer end of thecoil from the outer surface thereof, and tightening the rewound stripinto a compact cylinder.

13. The method of assembling a wound core on a form wound windingstructure comprising the steps of heat treating a flatwise spirallywound coil of magnetic strip material, the turns of which tightly engageeach other and the inside diameter of which is equal to the maximumdiameter of the portion of the winding structure to which it is to beapplied, to improve the magnetic qualities of the strip and give it apermanent set, threading the outer end of said magnetic strip throughthe window of the winding structure and forming a loop larger indiameter than the outside diameter of said heat treated coil and thenrotating the coil and 1001) while maintaining the end of the strip insubstantially fixed relation to the next underlying strip to unwind thecoil into the inside of the loop, said loop being of such size that thematerial of said strip is not strained beyond the elastic limit andlarge enough to permit it to move freely through the window in thewinding structure when all of the material has been wound into thelarger loop, and then collapsing said loop upon the winding structure sothat the completed core will have the same dimensions as the coil hadduring heat treatment and the turns in the completed core will have thesame sequence as in the heat treated coil.

14. The method of producing an assembled magnetic core and winding, saidmethod comprising the steps of unwinding a flatwise spirally wound coilof radially superposed turns of magnetic strip material, simultaneouslyrewinding said strip around one side of said winding into a loop largeenough to avoid binding on the winding sufficient to damage the windingor the strip and then collapsing said loop upon the winding so that thecompleted core thus formed will have substantially the same size as theorigi nal coil and will have the turns in the same sequence.

15. The method of producing an assembled magnetic core and winding, saidmethod comprising the steps of winding a strip of magnetic sheetmaterial flatwise into a coil of radially superposed turns, winding saidcoil around one side of said winding into a coil large enough to avoidbinding on the winding suflicient to damage the winding or the strip andtightening said coil to its original diameter.

16. The method of applying a flatwise spirally wound heat treated coilof magnetic material to a winding structure which comprises passing theouter end of said coil through the winding structure and back to thecoil to form a loop around one side of the winding structure, rotatingthe coil to unwind it and simultaneously rewind it around one side ofthe winding structure, securing the inner end of the rewound coil to thewinding structure and collapsing the turns of said rewound coil to forma compact magnetic core tightly surrounding the side of the windingstructure to which it has been applied, the turns of the completed corebeing in the same sequence as in the heat treated coil and the insideand outside diameters of the core being substantially the same as in theheat treated coil.

17. The method of producing an assembled magnetic core and windingstructure comprising the steps of winding a strip of magnetic sheetmaterial flatwise into a coil, passing the outer end of said coilthrough the winding structure and back to the'coil to form a loop aroundone side of the winding structure, rotating the coil to rewind it aroundone side of the winding structure, securing the end of the rewound stripagainstgmotion relatively to the winding structure, and tightening saidcoil to form a compact magnetic core tightly surrounding the side of thewinding structure.

18. The method of applying a coil of flatwise spirally wound magneticstrip material to a leg of a conductive winding structure whichcomprises relatively moving the coil and winding structure so asprogressively to link the convolutions of said coil around said leg andtransfer said leg from the outside to the inside ofsaid coil whilemaintaining sufllcient space between said leg and said strip to permitof free movement of said leg from the exterior to the interior of saidcoil without contact between said strip and said leg.

19. The method oi producing an assembled magnetic core and winding, saidmethod including the steps of winding a strip of magnetic sheet materialinto a coil of many superposed turns one outside the other, unwindingthe strip from said coil and simultaneously rewinding it around one sideof the winding with the turns in the same sequence as in the coil,maintaining during the operation a clearance between the strip and thewinding.

20. Themethod of producing an assembled magnetic core and winding, saidmethod including the steps of winding a strip of magnetic sheet materialinto a coil of many superposed turns one outside the other, unwindingthe strip from said coil and simultaneously rewinding it about one sideof the winding and the axis of the coil, maintaining during theoperation a clearance between the strip and the winding and tighteningthe rewound strip into a compact cylinder.

21. The method 01 iorming a core for an electromagnetic inductionapparatus comprising an electrical coil assembly and a core of thinstrip material'having a pronounced favorable magnetic orientationlongitudinally of the strip, which method comprises winding the stripflatwise into a coil of many turns, one above the other, heat treatingthe resulting coil to produce high permeability and a permanent set,unwinding the strip to break up adhesions and rewinding the strip so asto interlink it with the eiectrical coil assembly, keeping the bendingaction during unwinding and rewindlng always below that valuewhich wouldstrain the strip beyond the elastic limit of the material, and soconducting the rewinding operation that the resulting final coil will beof the same size which it had when it was heat treated, whereby elasticstrains in the rewound coil of strip are eliminated.

22. A transformer comprising a conductive winding including primary andsecondary coils of the form-wound type and a magnetic core substantiallyfilling the opening in the conductive winding, said core comprising amagnetic core element in the form of a spirally wound roll containingmany turns of strip material characterized by a permanent set due toheat treatment, said turns nesting flatwise tightly within one another,each turn being of substantially the size which it tends to assumebecause of the permanent set of the material due to heat treatment, saidturns passing many times through the opening in the conductive winding,said conductive winding substantially filling the opening in said rolland adapted to produce a magnetic flux flowing longitudinally of saidstrip.

Joan c. GRANFIELD.

